1. Field of the Invention
The present invention relates to a semiconductor device which is suitably used in display devices such as liquid crystal displays and to a method for producing the same.
2. Description of the Related Art
In liquid crystal displays, an active matrix type liquid crystal display device for displaying an image by switching by a thin film transistor (TFT) using an amorphous or polycrystalline silicon film formed on a glass or synthetic quartz substrate is the main current. At present, this liquid crystal panel is mainly driven by using a driver circuit which drives pixel transistors as independently set up externally. If this driver circuit could be constructed on the same substrate as in a pixel circuit of the liquid crystal display, it should become possible to obtain tremendous merits in view of the manufacturing costs of liquid crystal displays, reliability and so on.
However, at present, since crystallinity of a silicon film which constructs an active layer of TFT is poor, performance of TFT represented by mobility of a carrier is low so that it is difficult to prepare integrated circuits requiring high speed properties and high functionality, such as driver circuits. For the purpose of realizing TFT having a carrier with high mobility, in order to improve crystallinity of the silicon film, it is generally performed to thermally treat the silicon film with a laser.
The relationship between the crystallinity of the silicon film and the carrier mobility in TFT will be hereunder described. That is, a silicon film obtained by a laser heat treatment of an amorphous silicon film is generally a polycrystalline substance. In the grain boundary of the polycrystalline substance, a crystal defect is localized. This hinders the carrier movement of the active layer of TFT. Accordingly, in order to enhance the mobility in TFT, it is only required that not only the number of crossing the grain boundary during the movement of the carrier within the active layer is made small, but also a density of the crystal defect is made small. An object of the laser heat treatment is to form a polycrystalline silicon film which is large in crystal grain size and small in crystal defect in the grain boundary.
Next, a conventional production method of TFT will be described below. First of all, for example, a silicon oxide film is formed on a glass substrate by plasma CVD (chemical vapor deposition process). For example, an amorphous silicon film is deposited on this silicon oxide film by plasma CVD.
Subsequently, an excimer laser (XeCl (wavelength: 308 nm)) or a second harmonic of Nd:YAG laser (hereinafter referred to as “YAG2ω”) (wavelength: 532 nm) is irradiated on the amorphous silicon film. By this laser irradiation, a portion irradiated with a laser is molten. Thereafter, the molten silicon is crystallized with a decrease of the temperature, thereby forming a polycrystalline silicon film.
Thereafter, the polycrystalline silicon film is subjected to patterning. Next, a silicon oxide film and a metallic film (metallic film with low electrical resistance, such as Ta, Cr, and Mo) are formed on the polycrystalline silicon film.
Next, the metallic film is subjected to patterning, thereby forming a gate electrode. Next, N type or P type impurities are introduced into the polycrystalline silicon film by an ion doping process while making the gate electrode or a resist during forming the gate electrode act as a mask, thereby forming source and drain areas in a self-alignment manner. Thereafter, a silicon oxide film is deposited to form contact holes in the source, the drain and the gate, thereby accumulating a metallic film (for example, Al, W, and Mo). By subjecting this metallic film to patterning, wiring of the source, the drain and the gate is carried out. In this way, TFT of an n-channel type (NMOS) transistor is completed in a portion into which N type impurities are introduced, and TFT of a p-channel type (PMOS) transistor is completed in a portion into which P type impurities are introduced. An insulating film and a transparent electrode are further formed on this TFT, thereby obtaining a TFT panel. By using this TFT panel and further combining a liquid crystal, a polarized film, a color filter, etc. therewith, a liquid crystal display is completed.
As described previously, a TFT panel is formed by using a polycrystalline silicon film formed upon irradiation with a laser. At this time, an important point resides in the matter that for example, in the case of a TFT panel for mobile phone, at least the length of a beam on the elongated cross section of a laser is made longer than the short sides of the TFT panel. This is because in the case where the ends of beams are superposed by an ultraviolet laser beam such as an excimer laser, the characteristics are largely deteriorated in a superposed irradiation portion (superposing irradiation part) and the characteristics of TFT (for example, mobility and threshold voltage) are lowered (for example, see Morikawa et al., SID 04 DIGEST, (US), 2004, pp. 1088-1091). However, it is difficult for usual panels for television set to meet the foregoing conditions because the panel size is large.
In order to improve the characteristics of the foregoing superposing irradiation part, there is proposed a method in which a laser having a wavelength of from 390 nm to 640 nm is irradiated in a first area of an amorphous silicon film, thereby forming a first polycrystalline silicon film portion; and a laser having a wavelength of from 390 nm to 640 nm is then irradiated in an end of the first polycrystalline silicon film portion and a second area of the amorphous silicon film which comes into contact with the first polycrystalline silicon film portion, thereby forming a second polycrystalline silicon film portion so as to come into contact with the first polycrystalline silicon film portion (for example, see WO 02/31871, JP-A-2002-16015 and Morikawa et al., SID 04 DIGEST).
The reasons why a laser beam having the foregoing wavelength range is used are as follows.
In amorphous silicon films and polycrystalline silicon films, an absorption coefficient of laser is variously changed depending upon the wavelength. According to WO 02/31871, a wavelength of the laser is regulated at from 390 nm to 640 nm. As shown in FIG. 20, an absorption coefficient of a laser beam in the foregoing-wavelength area of 390 nm or more on a polycrystalline silicon film is not more than 60% of that of an amorphous silicon film. For that reason, if a laser is irradiated on an amorphous silicon film, whereby a polycrystalline silicon film is once formed, even when a laser having the foregoing wavelength area is again irradiated on this polycrystalline silicon film, the polycrystalline silicon film does not absorb energy of the laser beam so much as compared with the amorphous silicon film. As a result, the polycrystalline silicon film is not molten by re-irradiation, and its characteristics are not changed by the re-irradiation with a laser. For that reason, it is possible to obtain substantially homogenous characteristics over the whole of the polycrystalline silicon film.
In addition, as described previously, since the wavelength area of a laser beam is regulated at not more than 640 nm, as shown in FIG. 20, it is possible to secure an absorption coefficient of 10% or more of the amorphous silicon film. As a result, the amorphous silicon film is liable to absorb energy of a laser beam so that it becomes possible to easily achieve polycrystallization through melting.
Incidentally, as shown in FIG. 20, what the wavelength is from 500 nm to 550 nm is preferable because a difference in absorption coefficient between the amorphous silicon film and the polycrystalline silicon film becomes larger. What the wavelength is from 520 nm to 550 nm is more preferable because a difference in absorption coefficient between the amorphous silicon film and the polycrystalline silicon film becomes especially large.
In WO 02/31871, YAG2ω is used. Thus, as is clear from FIG. 20, even in the case of variously setting up the thickness of the silicon film, since the wavelength is 532 nm, the absorption coefficient in the polycrystalline silicon film is smaller than that of the amorphous silicon film.
As described previously, a pixel drive transistor of TFT is constructed of an NMOS transistor and a PMOS transistor. It is required to review how a mobility and a threshold voltage of a carrier in such transistors vary in the whole of the TFT panel. In a portion where a laser is irradiated twice (superposition part), the mobility is kept substantially uniform in all of the NMOS transistor and the PMOS transistor. Furthermore, even in the superposition part, the mobility is substantially equal to that in other portions.
In addition, even in all of the NMOS transistor and the PMOS transistor, the threshold voltage is substantially equal in all positions. This means that the threshold voltage is substantially equal in a portion where a laser is irradiated twice (superposition part) and a portion where a laser is irradiated only once.
As described previously, in an example of using a laser beam having a wavelength of 532 nm, since the wavelength of the laser beam is set up in an appropriate range, in a portion where a laser beam is irradiated once and a portion where a laser beam is irradiated twice, the mobility and the threshold voltage are uniform so that semiconductor devices with high quality can be provided.
However, when a TFT panel was actually prepared by using a laser beam having a wavelength of the foregoing preferred range, it became clear that a superposition area is delicately recognized by human eyes. This is because a human being recognizes a very delicate gradation difference. If the superposition part is recognized by human eyes, even when such does not affect display performance, there is a possibility that a commercial value is affected.